As mandated by many building codes, for new construction or substantial remodeling of old construction, the integrity of main and branch drains must be maintained to avoid leaks that result in sewer gas throughout the building or dwelling. In order to assure that the plumbing is sound and has been done to appropriate standards, it is a common test to cap off all of the branch drains as well as the main drain at the cleanout before it goes into the sewer system and to pour water into the stack that extends above the roof of the building.
It will be appreciated, however, that water testing is relatively dangerous in that one must deliver a supply of water to the stack so that the entire system is filled with water from at least six feet above the highest fixture.
As a matter of course, plumbers have to go out onto the roof with a hose and proceed to the point where the stack comes up through the roof. The plumber then has to fill up the stack until the entire drain system is filled with water. This is a dangerous exercise because one has to get onto the roof and squirt water into the stack, typically leaving the roof wet.
In the presence of the inspector, then one has to observe all of the plumbing seals from roof to basement after having capped off or plugged off all of the waste lines, be they sink waste, toilet pipes, shower waste, vanity waste or like waste lines. This is typically done by either capping the waste line at the fixture or by inserting an inflatable ball so as to seal off all of the waste pipe as well as the bottom-most part of the main drain.
The problem with water testing is that leaks do not necessarily show up immediately and may become evident as much as two to three weeks after the initial testing. Thus, while water is traditionally used to test main drains and their feeder lines, it is not a failsafe method to establish the integrity or water tightness of the main drain system.
There is a further problem with using water to test for leaks and that is a conservation problem. If one fills the main drain and branches with water and a leak is detected, then to carry out the repair, the system needs to be drained, the repair effected and the stack re-filled with water to test the repair. It will be appreciated that in a typical commercial building as much as 200 gallons of water is necessary to fill up the main drain and its branches per test. Multiple tests per day for a given community lead to undue wastage of water.
Some building codes permit the use of pressurized air to test drains, but the types of apparatus utilized for air testing of a main drain and its feeders requires complicated apparatus which is left in place and is rarely if ever used.
Thus, although airtight integrity is contemplated in many of the building codes, due to the difficulty and time consuming nature of performing airtight integrity tests and due to the elaborate apparatus necessary to perform such tests, this portion of the building code is largely ignored by building inspectors and plumbers alike.